Adult bull sharks can bite with a maximum force equivalent to 6,000N, a study of their jaws and jaw muscles has shown.

It is unclear why bull sharks have such strong bites, which are much greater than required to kill and eat prey.

Details of the discovery are published in the journal Zoology.

Maria Habegger of the University of South Florida in Tampa, US and colleagues in the US and Germany examined bite forces produced by 13 species of shark and their close relatives.

The species tested ranged from the 1m-long ratfish, a small relative of the sharks that scours the seabed for crabs and clams, to the great white shark, a large predatory shark that can approach 6m in length. Great white sharks feed on a variety of fish and marine mammals such as seals and dolphins.

Previous research has shown that large predatory sharks have very strong bites.

"We expect strong bite force values in the larger sharks that occupy top positions in the food chain, for example, the great hammerhead, great white shark, tigers and bull sharks," Ms Habegger told BBC Nature.

"These species usually prey upon large prey items such as dolphins, turtles and other sharks, so high bite forces are expected due to the mechanical demands of this type of prey."

But research has also shown that smaller species, such as ratfishes, have high bite forces for their size, perhaps due to their need to crush hard shells.

"So sometimes size is misleading. Although larger size sharks will exert higher values of bite force, the relative value of bite force is what matters, pound per pound how strong is the bite?," said Ms Habegger, who is studying for a PhD.

So Ms Habegger, her supervisor Dr Philip Motta, who has long studied the bite forces of sharks, and colleagues estimated the bite forces of the 13 species.

They dissected specimens to study their jaw muscles and worked out the forces these muscles can impart while closing the jaw.

The researchers then used mathematical techniques to remove the effect of body size, so they could make a fair comparison between species.

"What this study shows is that pound per pound bull sharks have the largest bite force value among all studied sharks," said the biologist.

"Bull sharks can bite harder than a great white shark and great hammerhead."

Dr Habegger says the research raises an intriguing question, however. Why do bull sharks need such a powerful bite?

The bull shark (Carcharhinus leucas) is a member of the requiem shark family, which includes the tiger and lemon shark. It prefers warm, shallow waters and is known to swim up rivers, often considerable distances, for example 2,220 miles (3,700 km) up the Amazon River in Peru, and over 1,800 miles (3,000 km) up the Mississippi River in Illinois.

Bull sharks can reach 3.5m long, and are so named for their large, stout heads, and pugnacious nature. The bite forces bull sharks impart change through their lifetime: smaller bull sharks actually bite harder than expected for their size, but larger individuals do not.

One idea is that this ability gives young bull sharks an advantage over other competing species; allowing them to eat more diverse prey earlier in their lives.

But overall, bull sharks, which the research shows can bite with a force of almost 6,000N at the back of the jaw and more than 2,000N at the front, seem to have bites that are too powerful.

"From our knowledge there is no need of such massive values to break fish skin or even to puncture bone," Ms Habegger told BBC Nature.

Strong jaws might be needed to crack turtle shells, as researchers still do not really understand how strong they are. Or it could be that a strong bite is particularly useful when hunting in murky waters, such as those that bull sharks inhabit.

"In a lower visibility environment catching prey may be more difficult than in open water. So once you get a prey between your jaws, securing it is crucial to not lose your meal."

The possibility remains, though, that the huge bite forces are simply an artefact of the large size these top predators can attain.